Stable crystalline (6R) -tetrahydrofolic acid

ABSTRACT

Pure and extremely stable crystalline (6S)- and (6R)-tetrahydrofolic acids, absolutely inert even when exposed to air and elevated temperature without stabilizers being added, are prepared by a crystallization process at a pH of ≧3.5 for the preparation of crystalline (6S)-tetrahydrofolic acid and at a pH of ≧2 for the preparation of crystalline (6R)-tetrahydrofolic acid.

BACKGROUND OF THE INVENTION

[0001] This invention relates to crystallineN-[4-[[(2-amino-1,4,5,6,7,8-hexahydro-4-oxo-(6S)- and-(6R)-pteridinyl)methyl] amino]benzoyl]-Lglutamic acid (termedcrystalline (6S)- or (6R)-tetrahydrofolic acid hereinbelow), to its use,and to a process for its preparation.

[0002] Tetrahydrofolic acid derivatives have 2 asymmetric centres. Sincethese derivatives are synthesized from folic acid, i.e.N-(pteroyl)-L-glutamic acid, the optically active C atom in the glutamicacid moiety is in the L form, while the optically active C atom in the6-position, which is usually formed by hydrogenation of the double bondin the 5,6-position of the pteroyl radical, exists in the racemic, i.e.(6R,S) form. Accordingly, synthetic tetrahydrofolic acid derivatives arecomposed of a 1:1 mixture of 2 diastereomers.

[0003] Tetrahydrofolates are mainly used in the form of calcium5-formyl-5,6,7,8-tetrahydrofolate (leucovorin) or calcium5-methyl-5,6,7,8-tetrahydrofolate as pharmaceuticals for the treatmentof megaloblastic folic acid anaemia, as an antidote for improving thetolerance of folic acid antagonists, specifically aminopterin andmethotrexate in cancer therapy (“antifolate rescue”), for enhancing thetherapeutic effect of fluorinated pyrimidines and for the treatment ofautoimmune diseases such as psoriasis and rheumatic arthritis, forimproving the tolerance of certain antiparasitics, such astrimethoprim-sulfamethoxazole, and for reducing the toxicity ofdideazatetrahydrofolates in chemotherapy. Tetrahydrofolic acid is alsoused as starting material for the preparation of a variety oftetrahydrofolic acid derivatives.

[0004] To date, the direct use of tetrahydrofolic acid as apharmaceutically and as a starting material for the preparation of avariety of tetrahydrofolic acid derivatives was made impossible by thedifficulty encountered when preparing tetrahydrofolic acid in a puritywhich is acceptable for a pharmaceutical active substance and by theextreme instability of tetrahydrofolic acid, in particular itspronounced sensitivity to oxidation [see, in this context, also A. L.Fitzhugh, Pteridines 4(4), 187-191 (1993)]. Various methods weredeveloped to overcome this instability, and particular mention must bemade in connection with the present invention of DE-OS 2 323 124.Specific mention must also be made of EP 600 460 in the context ofprocesses for the preparation of tetrahydrofolic acid and in connectionwith the present invention. However, no process which is feasible on anindustrial scale has been found to date for the preparation ofultrapure, sufficiently stable tetrahydrofolic acid which would allowthe pharmaceutical application of tetrahydrofolic acid.

SUMMARY OF THE INVENTION

[0005] Surprisingly, it has been found that chemically and opticallyultrapure (6S) or (6R) tetrahydrofolic acid with an outstandingstability can be obtained by crystallizing optically pure (6S)- oroptically pure (6R)-, enriched (6S)- or enriched (6R)- or else(6R,S)-tetrahydrofolic acid. The resulting crystalline (6S)- and/or(6R)-tetrahydrofolic acid allows for the first time the use of thesubstance as a pharmaceutical or as a starting material for theindustrial-scale preparation of other ultrapure tetrahydrofolic acidderivatives.

[0006] (6S)-tetrahydrofolic acid is crystallized from a polar medium ata pH of ≧3.5, while (6R)-tetrahydrofolic acid is crystallized from apolar medium at a pH of ≧2.

[0007] Suitable polar media include but are not limited to: especiallywater or a mixture of water and an organic solvent which is misciblewith water, such as water-soluble alcohols, for example methanol,ethanol, n-propanol, iso-propanol, ethylene glycol, a water-solublelower aliphatic carboxylic acid, for example formic acid, acetic acid,lactic acid, or water-soluble amides, for example formamide,dimethylformamide, dimethylacetamide, 1-methylpyrrolidone,2-methylpyrrolidone, 2-piperidinone. No particular restrictions apply tothe nature of the solvent employed and the mixing ratio, sincecrystalline (6S)-tetrahydrofolic acid and crystalline(6R)-tetrahydrofolic acid in general have lower solubilitycharacteristics than the corresponding amorphous forms.

[0008] To initiate crystallization of (6S)-tetrahydrofolic acid, a pH ofbetween 3.5 and 6.5 is particularly suitable. To initiatecrystallization of (6R)-tetrahydrofolic acid, a pH of between 2 and 5.5is particularly suitable. The optimum pH for initiating crystallizationdepends on the materials employed and the intended object and can bedetermined by simple experiments. In general, a higher salt content inthe starting solution will require a lower pH for initiatingcrystallization, and a lower pH for initiating crystallization requiresa slower crystallization process since otherwise amorphoustetrahydrofolic acid precipitates at a pH of around 3. For example,direct crystallization of (6S)-tetrahydrofolic acid from a reactionsolution obtained by reducing folic acid using borohydride strictlyrequires a pH of ≦4.8 for initiating crystallization. Aftercrystallization has been initiated, the pH may be varied.

[0009] During the crystallization of (6S)-tetra-hydrofolic acid and alsoduring the crystallization of (6R)-tetrahydrofolic acid, the pH rises ormay be kept constant by adding an acid or buffer. In the case of thecrystallization of (6S)-tetrahydrofolic acid, a pH of between 4.5 and5.5 during the crystallization is preferred if it is intended tooptically enrich (6S) tetrahydrofolic acid, while a pH of between 3.5and 4.5 during the crystallization is preferred if it is intended toprepare stable crystalline (6S)-tetrahydrofolic acid. In the case of thecrystallization of (6R)-tetrahydrofolic acid, a pH of between 3.5 and4.5 during the crystallization is preferred independently of the desiredresult. The crystallization can be carried out in each case at roomtemperature, at elevated temperature or else at reduced temperature.

[0010] The time required for crystallization varies between a fewminutes and several days. As a rule, longer crystallization times resultin higher purity and more stable products.

[0011] (6S)- and (6R)-tetrahydrofolic acid crystallize spontaneously byslowly adjusting the pH, either starting from a pH which is lower thanthe pH suitable for initiating the crystallization of the isomer inquestion, or, preferably, starting from a higher pH. Crystallization maybe triggered by seeding with the corresponding crystallinetetrahydrofolic acid in the pH range which is suitable for initiatingcrystallization of the isomer in question.

[0012] The starting material for the crystallization can be racemic(6R,S)-tetrahydrofolic acid, enriched (6S)- or (6R)-tetrahydrofolic acidas well as amorphous or crystalline (6S)- or (6R)-tetrahydrofolic acid.Suitable as starting material are not only isolated solid substances,such as, for example, (6R,S)-tetrahydrofolic acid, addition salts ofsulphuric and sulphonic acid with (6S)-tetrahydrofolic acid prepared asdescribed in EP 495 204, as well as tetrahydrofolic acid which has beenprepared in situ from folic acid by catalytic hydrogenation or byreduction using boron hydride. (6R) tetrahydrofolic acid may becrystallized directly from the (6S)-tetrahydrofolic acid crystallizationmother liquor. Both isomers may be crystallized either from a solutionobtained, for example or by bringing the pH to >7 or <2, or from asuspension.

[0013] By using amorphous or partially crystalline optically puretetrahydrofolic acid or salts thereof as the starting material for thecrystallization, the above described process yields crystallinetetrahydrofolic acid of previously unachieved purity (>98%) and,equally, previously unachieved stability.

[0014] The invention also relates to the use of crystalline (6S)- and/or(6R)-tetrahydrofolic acid as a component for the preparation ofpharmaceuticals or for the preparation of other tetrahydrofolic acidderivatives since the quality of crystalline (6S)- and(6R)-tetrahydrofolic acid in solid form remains high over a virtuallyunlimited period due to its outstanding stability. The invention alsorelates to pharmaceutical preparations comprising crystalline (6S)-and/or (6R)-tetrahydrofolic acid. The pharmaceutical preparation is madeby known processes, such as, for example, lyophilization. The uses andmethods of use are analogous to those of known substances from the fieldof the tetrahydrofolates, such as, for example,5-formyl-5,6,7,8-tetrahydrofolic acid.

[0015] The invention furthermore relates to a process for separating(6R,S)-tetrahydrofolic acid by fractional crystallization to give thetwo diastereomers (6S)- and (6R)-tetrahydrofolic acid. This process isvery simple and high-yielding. Even upon the first crystallization of acrude racemic (6R,S)-tetrahydrofolic acid, crystalline(6S)-tetrahydrofolic acid is obtained in yields of above 70% while its(6S) component amounts to above 75%, and crystalline(6R)-tetrahydrofolic acid is obtained in yields of above 50% while its(6R) component amounts to above 80%. Further crystallization steps underanalogous conditions allow crystalline (6S)- and (6R)-tetrahydro-folicacid with an isomeric purity of above 95% to be obtained.

[0016] (6R)- or (6S)-tetrahydrofolic acid can also be used directlywithout isolation for the preparation of other tetrahydrofolic acidderivatives. For example, enriched 5,10-methylene-(6S)-tetrahydrofolicacid can be prepared very easily by adding formaldehyde to a(6R)-tetrahydrofolic acid solution.

[0017] Without further elaboration, it is believed that one skilled inthe art can, using the preceding description, utilize the presentinvention to its fullest extent. The following preferred specificembodiments are, therefore, to be construed as merely illustrative, andnot limitative of the remainder of the disclosure in any way whatsoever.

[0018] In the foregoing and in the following examples, all temperaturesare set forth uncorrected in degrees Celsius and unless otherwiseindicated, all parts and percentages are by weight.

[0019] The entire disclosures of all applications, patents andpublications, cited above and below, and of corresponding Swissapplication 01442/94-6, are hereby incorporated by reference.

[0020] The tetrahydrofolic acid contents and the isomer contents givenin the following examples were in each case determined by HPLC. Alltetrahydrofolic acid contents are based on the anhydrous substance.

EXAMPLE 1 Stabilities

[0021] To determine the stability of crystalline (6S)- and(6R)-tetrahydrofolic acid, the substances together with comparisonsamples were stored under severe conditions at 60° C. in the air. Theremaining tetrahydrofolic acid content was measured at periodicintervals and is shown in comparison with the initial value. Test periodin days at 60° C. in air 0 2 6 13 21 28 57 360 Crystalline 100.0% 100.1%102.5% 98.7% 103.6% 103.1% 101.2% 93.3% (6S)-tetra- hydrofolic acidCrystalline 100.0% 96.4% 96.1% 93.3% 92.7% 82.0% (6R)-tetra- hydrofolicacid “Yamanouchi's 100.0% 83.6% 48.6% 31.0% 13.4% crystalline(6R,S)-tetra- hydrofolic acid” Amorphous 100.0% 60.4% 13.7% 7.9%(6S)-tetra- hydrofolic acid amorphous 100.0% 70.5% 29.1% 21.6% 9.8%(6R)-tetra- hydrofolic acid amorphous 100.0% 53.4% 17.4% 13.2%(6R-,S)-tetra- hydrofolic acid

[0022] Even after a prolonged test period at 60° C. in the air,crystalline (6S)- and (6R)-tetrahydrofolic acid remain very pale, almostwhite. In contrast, the other products which are included for comparisonreasons discolor rapidly to a high degree.

[0023] The substances employed for the stability tests were prepared asfollows:

[0024] crystalline (6S)-tetrahydrofolic acid as in Example 6 of thepresent patent application

[0025] crystalline (6R)-tetrahydrofolic acid as in Example 9 of thepresent patent application

[0026] “Yamanouchi's crystalline (6R,S)-tetrahydrofolic acid” as inDE-OS 2 323 124, Example 3

[0027] amorphous (6S)-tetrahydrofolic acid (6S)-tetrahydrofolic acid isdissolved in acetic acid and precipitated using diethyl ether

[0028] amorphous (6R)-tetrahydrofolic acid (6R)-tetrahydrofolic acid isdissolved in acetic acid and precipitated using diethyl ether

[0029] amorphous (6R,S)-tetrahydrofolic acid (6R,S)-tetrahydrofolic acidis dissolved in acetic acid and precipitated using diethyl ether.

EXAMPLE 2 Powder X-ray Diagrams

[0030] To characterize the structural properties (crystallinity) of thecrystalline (6S)- and (6R)-tetrahydrofolic acid, powder X-ray diagrams(diffraction spectra) were recorded of these substances together withcomparison samples under identical conditions.

[0031] Crystalline (6S)- and crystalline (6R)-tetrahydrofolic acid bothresult in neatly resolved, differing spectra with sharp bands and a lowdegree of background. The spectra suggest high contents of crystallinematter. In contrast, “Yamanouchi's crystalline (6R,S)-tetrahydrofolicacid” results in a poorly resolved spectrum with fuzzy bands (diffusemaxima) and a high degree of background. This spectrum suggestspredominantly amorphous (6R,S)-tetrahydrofolic acid and only a lowpercentage of crystalline matter.

[0032] Substances used for producing the powder X-ray diagrams wereprepared as follows:

[0033] crystalline (6S)-tetrahydrofolic acid as in Example 6 of thepresent patent application

[0034] crystalline (6R)-tetrahydrofolic acid as in Example 9 of thepresent patent application

[0035] “Yamanouchi's crystalline (6R,S)-tetrahydrofolic acid” as inDE-OS 2 323 124, Example 3

EXAMPLE 3

[0036] a) 4 g of (6R,S)-tetrahydrofolic acid are suspended in 16 ml ofwater and the pH is brought to 9 using 25% ammonia. At 50° C., theresulting solution is brought to pH 5 using hydrochloric acid and thenslowly to the desired pH using sodium hydroxide solution. 2 ml aliquotsare sampled at the pH indicated, filtered with suction and washed with asmall amount of water. Percentage of pH Quantity (6S) pH 5.5 0.03 g87.8% pH 6.0 0.06 g 87.8% pH 6.4 0.02 g 88.6%

[0037] b) 4 g of (6R,S)-tetrahydrofolic acid are suspended in 16 ml ofwater and brought to pH 9 using 25% ammonia. At 50° C., the resultingsolution is brought to pH 5 using hydrochloric acid and then slowly tothe desired pH using hydrochloric acid. 2 ml aliquots are sampled at thepH indicated, filtered with suction and washed with a small amount ofwater. Percentage of pH Quantity (6S) pH 4.8 0.09 g 72.7% pH 4.5 0.15 g57.9% pH 4.2 0.27 g 51.8%

[0038] c) 4 g of (6R,S)-tetrahydrofolic acid are suspended in 16 ml ofwater and brought to pH 9 using 25% ammonia. At 50° C., the resultingsolution is brought to pH 5 using hydrochloric acid and then slowly tothe desired pH, again using hydrochloric acid. 2 ml aliquots are sampledat the pH indicated, filtered with suction and washed with a smallamount of water. Percentage of pH Quantity (6S) pH 4.1 0.16 g 56.2% pH3.8 0.10 g 52.2% pH 3.5 0.22 g 51.8% pH 3.0 0.12 g 51.6%

[0039] d) 10 g of (6R,S)-tetrahydrofolic acid are suspended in 80 ml ofwater and brought to pH 1.3 using 1N hydrochloric acid. At roomtemperature, the resulting solution is slowly brought to the desired pHusing 1.8 N ammonia. 2 ml aliquots are sampled at the pH indicated,filtered with suction and washed with a small amount of water.Percentage of pH Quantity (6S) pH 2.0 0.03 g 50.3% pH 2.3 0.13 g 50.5%pH 2.5 0.12 g 49.3% pH 2.8 0.22 g 50.8% pH 3.1 0.17 g 49.5% pH 3.5 0.21g 51.5% pH 4.0 0.14 g 59.1% pH 4.5 0.16 g 56.1% pH 5.1 0.22 g 72.7% pH5.5 0.20 g 70.9%

[0040] The process parameters of the data listed in Tables a) to d) arenot optimal since all experiments were carried out following the sameprotocol to improve their comparability.

EXAMPLE 4

[0041] 5 g aliquots of (6R,S)-tetrahydrofolic acid are suspended in 50ml of water and allowed to stand for days at room temperature or at 40°C. After filtration with suction (filtration temperature=crystallisationtemperature) and washing, the following results are obtained: RT 40° C.percentage Percentage Quantity of (6S) Quantity of (6S) pH 3.1¹⁾ 4.2 g52.5% 4.5 g 52.2% pH 4.2²⁾ 3.5 g 58.9% 3.9 g 59.3% pH 5.1³⁾ 1.8 g 82.1%1.5 g 81.0%

[0042] The process parameters of the data listed in the table are notoptimal since all experiments were carried out following the sameprotocol to improve their comparability.

EXAMPLE 5

[0043] 40 g of (6R,S)-tetrahydrofolic acid are suspended in 160 ml ofwater and brought to pH 9.3 using 25% ammonia. At 50° C., the resultingsolution is slowly brought to pH 5.1 using hydrochloric acid, and the pHis kept at between 5.1 and 5.2 during the following crystallizationphase. When the crystallization has ended, the mixture is cooled to 0-5°C., filtered under pressure and washed with water.

[0044] This gives 19 g of crystalline (6S)-tetrahydrofolic acid with achemical content of 95.9% and a (6S) percentage of 80.5.

[0045] One half of the mother liquor is precipitated with 1.1 g ofethanol, giving enriched amorphous (6R) tetrahydrofolic acid with achemical content of 63.3% and a (6R) percentage of 75.9, while in theother half of the mother liquor the pH is rapidly brought to 3.5 using6.3 g of hydrochloric acid, resulting in enriched, amorphous(6R)-tetrahydrofolic acid with a chemical content of 64.8% and a (6R)percentage of 75.9.

EXAMPLE 6

[0046] 60 g of an addition salt of (6S)-tetrahydrofolic acid withbenzene sulphonic acid with a (6S) percentage of 99.9, prepared asdescribed in EP 495 204, are suspended in 240 ml of water, and the pH ofthe suspension is brought to 5.5 using 63 ml of 1.8 N ammonia or 55.2 mlof 2N sodium hydroxide solution. A pH of 5.6 is maintained. The white,thick suspension is subsequently brought to pH 9.3 using 30% sodiumhydroxide solution, and the resulting clear solution is heated at 50° C.

[0047] The pH is subsequently slowly brought to 5.2 using hydrochloricacid, and then, after the mixture has been seeded with crystalline(6S)-tetrahydrofolic acid, 43.0 g of crystalline (6S)-tetrahydrofolicacid with a chemical content of 96.8% and a (6S) percentage of 99.9 areobtained.

[0048] By dissolving 40 g of the resulting crystalline(6S)-tetrahydrofolic acid in 160 ml of water at pH 9 and subsequentlyslowly bringing the pH to 4.2, using hydrochloric acid, 32.5 g ofcrystalline (6S)-tetrahydrofolic acid with a chemical content of 98.5%and a (6S) percentage of 100.0 are obtained after seeding withcrystalline (6S)-tetrahydrofolic acid.

[0049] Further recrystallizations at pH 4.2 give crystalline(6S)-tetrahydrofolic acid with a chemical content of >99% and a (6S)percentage of 100.0.

[0050] The solubility of the resulting crystalline (6S) tetrahydrofolicacid in water is 0.0022% at room temperature.

EXAMPLE 7

[0051] 40 g of (6R,S)-tetrahydrofolic acid are suspended in 160 ml ofwater and 40 ml of methanol and the suspension is brought to pH 9.1using 25% ammonia. At 50° C., the resulting solution is slowly broughtto pH 5.1 using hydrochloric acid, and a pH of between 5.1 and 5.2 ismaintained during the subsequent crystallization phase. When thecrystallization has ended, a 20 ml sample is filtered off with suctionat 50° C. and washed with water/methanol. This gives 1.3 g ofcrystalline (6S) tetrahydrofolic acid with a chemical content of 96.1%and a (6S) percentage of 83.0.

[0052] The main portion is cooled to 0-5° C., filtered under pressureand washed using water/methanol. This gives a further 18.6 g ofcrystalline (6S)-tetrahydrofolic acid with a chemical content of 90.9%and a (6S) percentage of 67.1.

EXAMPLE 8

[0053] 60 g of folic acid are suspended in 240 ml of water and broughtto pH 11.5 using a 30% sodium hydroxide solution. The resulting solutionis reduced at 70° C. using 30 g of sodium borohydride in 120 ml of waterand 12 g of 30% sodium hydroxide solution. After a reaction time ofapproximately 5 hours, the reaction mixture is diluted with 180 ml ofwater and slowly brought to pH 4.5 using hydrochloric acid. During thesubsequent crystallization phase, the pH rises to approximately 5.5. Thesuspension is filtered under pressure at 0-5° C. and washed with a smallamount of water.

[0054] This gives 25.5 g of crystalline (6S)-tetrahydrofolic acid with achemical content of 94.4% and a (6S) percentage of 82.7. The watercontent after drying is 4.0%.

[0055] By dissolving 20 g of the resulting crystalline(6S)-tetrahydrofolic acid in 80 ml of water at pH 9 and subsequentlyslowly bringing the pH to 5.1 using hydrochloric acid, 4.5 g ofcrystalline (6S)-tetrahydrofolic acid with a chemical content of 94.0%and a (6S) percentage of 94.7 are obtained after seeding withcrystalline (6S)-tetrahydrofolic acid. The water content after drying is1.8%.

EXAMPLE 9

[0056] 50 g of amorphous (6R)-tetrahydrofolic acid with a (6R)percentage of 99.4 are suspended in 600 ml of water and the suspensionis brought to pH 9.0 using 25% ammonia. The resulting clear solution isheated at 50° C.

[0057] After subsequently slowly bringing the pH to 4.4 usinghydrochloric acid and maintaining this value, 42.0 g of crystalline(6R)-tetrahydrofolic acid with a chemical content of 96.2% and a (6R)percentage of 99.5 are obtained.

[0058] The solubility of the resulting crystalline (6R)-tetrahydrofolicacid in water is 0.014% at room temperature.

[0059] Further recrystallizations at pH 4.4 give crystalline(6R)-tetrahydrofolic acid with a chemical content of >98% and a (6R)percentage of >99.5.

EXAMPLE 10

[0060] 40 g of (6R,S)-tetrahydrofolic acid are suspended in 160 ml ofwater and the suspension is brought to pH 9.3 using 25% ammonia. At 50°C., the resulting solution is slowly brought to pH 5.1 usinghydrochloric acid, and a pH of between 5.1 and 5.2 is maintained duringthe following crystallization phase. After crystallization has ended,the mixture is cooled to room temperature, filtered under pressure andwashed with water.

[0061] 18.2 g of crystalline (6S)-tetrahydrofolic acid with a chemicalcontent of 94.0% and a (6S) percentage of 77.8 are obtained.

[0062] The mother liquor from the (6S)-tetrahydrofolic acidcrystallization step is reheated to 50° C. and slowly brought to pH 4.4using hydrochloric acid, and the pH is maintained between 4.0 and 4.5during the subsequent crystallization of (6R)-tetrahydrofolic acid.After crystallization has ended, the mixture is cooled to roomtemperature, filtered under pressure and washed with water.

[0063] 12 g of crystalline (6R)-tetrahydrofolic acid with a chemicalcontent of 78.0% and a (6R) percentage of 74.8 are obtained.

EXAMPLE 11

[0064] Starting from racemic tetrahydrofolic acid, the process describedwas repeated at pH 5.2 and 45° C. under conditions analogous to thosedescribed in EP 600 460, Example 2. The resulting products were examinedfor chemical and optical purity. The chemical total yield was alsorecorded at each step. Purity Percentage Percentage Content of 6S Totalof 6S in g/g content yield Starting 50% 89.6% 44.8% 100% material Run 180.5% 96.9% 78.0% 53.5% Run 2 90.0% 97.3% 87.6% 45.4% Run 3 94.4% 97.4%91.9% 42.0% Run 4 96.4% 96.7% 93.2% 38.9% Run 5 97.7% 96.2% 94.0% 35.2%Run 6 98.4% 95.6% 94.1% 32.2% Run 7 98.9% 96.0% 94.9% 28.6%

[0065] As can be seen clearly from these data, the repeated applicationof this process allows the percentage of (6S) in the process product tobe increased while a decrease in product content, caused by repeatedlycarrying out the process, also occurs. Thus, a product with a purity ofover 98% was not prepared even by repeated application of this process.

[0066] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

[0067] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. A member selected from the group consisting ofcrystalline (6S)-tetrahydrofolic acid and crystalline(6R)-tetrahydrofolic acid, said member having a purity of more than 98%.2. Crystalline (6S)-tetrahydrofolic acid according to claim 1 , saidmember having a purity of more than 98%.
 3. Crystalline(6R)-tetrahydrofolic acid according to claim 1 , said member having apurity of more than 98%.
 4. A process for the preparation of crystalline(6S)-tetrahydrofolic acid, characterized in that (6S)- or 10(6R,S)-tetrahydrofolic acid is crystallized in a polar medium at a pH of≧3.5.
 5. A process for the preparation of crystalline(6R)-tetrahydrofolic acid, characterized in that (6R)- or(6R,S)-tetrahydrofolic acid is crystallized in a polar medium at a pH of≧2.
 6. A process according to claim 4 , in which water or a mixture ofwater and a polar water-soluble organic solvent is used as the polarmedium and crystallization is effected from a solution or a suspension.7. A process according to claim 5 , in which water or a mixture of waterand a polar water-soluble organic solvent is used as the polar mediumand crystallization is effected from a solution or a suspension.
 8. Aprocess according to claim 4 , wherein the pH is brought to between 3.5and 6.5 to initiate crystallization of (6S)-tetrahydrofolic acid.
 9. Aprocess according to claim 5 , wherein the pH is brought to between 2and 5.5 to initiate crystallization of (6R)-tetrahydrofolic acid.
 10. Aprocess according to claim 4 , characterized in that the pH ismaintained constant during crystallization.
 11. A process according toclaim 4 , wherein (6S)-tetrahydrofolic acid is separated off byfractional crystallization and (6R)-tetrahydrofolic acid is isolatedfrom the remaining mother liquor.
 12. A process according to claim 4 ,wherein a pH of between 4.5 and 5.5 is maintained during crystallizationin order to optically enrich the product.
 13. A process according toclaim 4 , wherein a pH of between 3.5 and 4.5 is maintained duringcrystallization in order to prepare stable crystalline(6S)-tetrahydrofolic acid.
 14. A process according to claim 5 , whereina pH of between 3.5 and 4.5 is maintained during crystallization.
 15. Amember according to claim 2 , having a purity of above 99%.
 16. A memberaccording to claim 3 , having a purity of above 99%.
 17. Crystalline(6S)-tetrahydrofolic acid according to claim 2 , having a content of atleast 98.7% after being stored in air at 60° C. for 57 days. 18.Crystalline (6R)-tetrahydrofolic acid according to claim 3 , having acontent of at least 92.7% after being stored in air at 60° C. for 21days.
 19. A pharmaceutical preparation comprising a member according toclaim 1 .
 20. A pharmaceutical preparation comprising a member accordingto claim 15 .
 21. A pharmaceutical preparation comprising a memberaccording to claim 17 .